In the manufacturing process of semiconductor integrated circuits, etching using low pressure plasma or the like is performed to form very small holes and grooves in a semiconductor substrate such as a silicon wafer. Conventionally, in an etching process, the etching is performed after first masking, by a resist film, the areas of a substrate where no holes or grooves are to be formed. As a result, only the areas which have not been masked are selectively etched away, allowing holes and grooves of arbitrary shape to be formed upon removing the resist film after etching. The depth of holes and grooves formed in this way depends on various parameters, such as the etching time, the gas type, gas pressure, etc., and thus, to make the depth of a hole or groove to a target depth, control is performed whereby the etching stop point is determined and the parameters are adjusted while monitoring the actual depth in-process.
Furthermore, contrary to the case of etching, film growth processes are also carried out, in which an oxide film, metal film or the like is formed in the areas which have not been masked, and in such cases as well, it is necessary to monitor the thickness of the film growth area in order to determine the end point of the process.
Various proposals have been made in the prior art with regard to technology for optically measuring the depth and level difference of micro-holes formed by etching, the film thickness of a film layer to be removed, the thickness of a substrate or liquid crystal whereof the surface is to be gradually removed by polishing or the like, the thickness of a grown film, and so forth, as will be described below.
Patent Literatures 1 through 3 disclose a device which acquires interference spectrum data by spectrometric measurement of interference light generated through interference between reflected light from the bottom of the hole or groove to be measured and reflected light from the perimeter of the hole or the top edges of the groove, or interference light generated through interference between reflected light from the top surface of the substrate, which is to be measured, and reflected light from the bottom surface of the substrate; analyzes interference fringes through spectral fitting, and computes the hole or groove depth or the substrate or film layer thickness based on those interference fringes.
Furthermore, Patent Literature 4 discloses finding the film thickness by acquiring interference spectrum data through spectrometric measurement of interference light generated through interference between reflected light from two surfaces of the film layer to be measured, and performing a Fourier transform on the spectrum.
Furthermore, Patent Literatures 5 and 6 disclose checking the progress of a process by applying time differentiation to an interference spectrum acquired through spectrometry, and comparing the calculated time differential spectrum and a past reference spectrum, which has been determined in advance, which satisfied the desired process parameters.
Patent Literature 7 describes the technology of detecting the point at which etching has advanced by λ/4 by irradiating an etched structure with a laser of wavelength λ and detecting the maximum and minimum interference amplitude of the reflected waves.